I had a similar problem many years ago using a clay with a high
Staffordshire fireclay content. I was bisque firing at about 950oC and
getting very fine cracks in the ware which opened up in the stoneware
glaze firing. Turned out that at that temperature bisque the clay could
not withstand the shock of its own cooling. The answer was either to
bisque at 1000oC or soak the 950 bisque for about 35 minutes. I chose
the former and it cured the problem.

I notice you are using a UK ISP so this might have some relevance to
you.

What clay are you using and how high the bisque?

Steve
Bath
UK


In article , JM
writes
Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled as
the pots.

Recently my ware has developed fine hairline cracking during bisque firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted all
known possibilities.

JM



--
Steve Mills
Bath
UK